Method for measurement of silanol group concentration and cell for measurement

ABSTRACT

The removal of intra-cell moisture can be performed within a short period of time by the use of pressure cell. In particular, a method of measuring the concentration of silanol groups characterized in that in the measurement of the concentration of silanol groups in a silicon compound according to infrared absorption spectroscopy, a step of maintaining an intra-cell pressure at 20 Pa or below and a step of maintaining the same at 0.2 to 1 MPa are repeated at least twice before charging the cell with a silicon compound, thereafter a silicon compound is introduced in the cell and cell for measurement thereof is carried out to thereby identify the concentration of silanol groups in the silicon compound. For this method, there is provided a cell for infrared absorption spectroscopy measurement thereof is carried out to thereby identify the concentration of silanol groups in the silicon compound. For this method, there is provided a cell for infrared absorption spectroscopy measurement which can withstand a vacuum of 20 Pa or below and also can withstand a pressurization of 0.2 to 1 MPa.

TECHNICAL FIELD

Silicon halide compounds such as silicon tetrachloride and the like arehighly reactive with water and react easily with a very small amount ofwater in the air to form hydrogen chloride gas and a silanol group. Inthe present invention, infrared absorption spectrum is utilized forquantitative determination of a very small amount of silanol groupcontained in a silicon compound. Further, in the present invention, apressure-resistant cell for measurement of infrared absorption spectrumis used, enabling a rapid and highly accurate measurement of silanolgroup concentration.

BACKGROUND ART

Silicon halide compounds suitably used as a material for formation ofsilicon nitride film, undergo hydrolysis by mere contact with a slightamount of water vapor contained in the air, to form hydrogen chloridegas and a silanol group. It is known that the thus-formed silanol groupgives an adverse effect on the property of the silicon nitride filmformed from a silicon halide compound. Therefore, it is necessary toquantitatively determine the silanol group of a silicon halide compoundfor quality control of the compound.

In order to quantitatively determine a very small amount of the silanolgroup present in a silicon compound, infrared spectrophotometry isknown. For example, in JP-A-9-318525 is described an infraredspectrophotometry wherein an optical path length, i.e. a length ofsample layer through which an infrared light is transmitted, is set at50 to 150 mm in order to measure a silanol group of about 0.1 ppm. Inthe method for measurement of silanol group, described in theliterature, there is used a cell constituted by a stainless steel-madecylinder and infrared-transmitting, calcium fluoride-made windows fittedto the cylinder.

In order to measure a silanol group of a very small amount, for example,0.1 ppm or less, it is generally necessary to remove a very small amountof the water adhering to a cell to be used for measurement and thenplace a sample into the cell. It is because, when a sample is placed ina cell of insufficient water removal and is measured for theconcentration of silanol group, the residual water in the cell andsilicon halide compound react with each other to form a silanol group,which is included in the measurement data obtained, as an error. Hence,in the method for measurement of silanol group, described in theJP-A-9-318525, it was necessary to beforehand pass a large amount ofnitrogen gas of water content of 0.5 ppm or less, through the cellinside, or, in addition with the passing of the above mentioned nitrogengas, to clean the cell inside with the sample per se. When the sample isa silicon halide of low volatility, such as hexachlorodisilane or thelike, the above-mentioned cell cleaning has had to be conducted taking avery long time when a new sample is filled in the cell in place of theused sample.

As described above, with the conventional method, much time and laborhave been required for the removal of water from cell inside, makingdifficult the rapid measurement of a very small amount of silanol group.

The present invention is intended to provide a method which can measurea silanol group of very small amount (0.05 to 0.1 ppm level) rapidly andeasily, and a cell for measurement of infrared absorption spectrum, usedin the method.

DISCLOSURE OF THE INVENTION

The present inventors made a study in order to achieve theabove-mentioned tasks. As a result, by using a cell for measurement ofinfrared absorption spectrum, which can withstand a reduced pressure andalso an applied pressure, and repeatedly conducting pressure reductionof cell inside and pressure application of cell inside using a dry inertgas, water removal from cell inside has been made possible in very shorttime.

The first aspect of the present invention lies in:

a method for measurement of silanol group concentration in siliconcompound by infrared spectrophotometry, which comprises:

conducting, prior to filling of a silicon compound in a cell, at leasttwice each of a step of keeping the cell inside at 20 Pa or lower and astep of keeping the cell inside at 0.2 to 1 MPa,

then, introducing the silicon compound into the cell and measuring theinfrared absorption spectrum thereof, to measure the concentration ofthe silanol group in the silicon compound.

The second aspect of the present invention lies in:

a cell used for measurement of infrared absorption spectrum, which canwithstand a reduced pressure of 20 Pa or lower and a pressure of 0.2 to1 MPa. Incidentally, Pa and MPa are each a pressure unit and mean Pascaland mega Pascal, respectively. They are in a relation of 1 MPa=10⁶ Pa.

The present invention is described in detail below.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a vertical sectional view showing an example of the cell formeasurement of infrared absorption spectrum (this cell is hereinafterreferred to simply as cell), used in the present invention.

FIG. 2 is a front view of the cell of FIG. 1.

FIG. 3 shows vertical sectional views of the parts obtained bydisassembling the cell of FIG. 1.

FIG. 4 is a conceptual view indicating the connections between a celland pipes running in the vicinity of the cell, which are necessary forfeeding a sample into the cell.

Explanation of the Symbols Used in FIG. 1 to FIG. 3

-   -   1: Cell trunk (hereinafter referred to simply as trunk)    -   2: Pipe for connection to vacuum system    -   3: Pipe for connection to sample system    -   4,5: Infrared-transmitting window panels    -   6,7: Holders for window panel    -   8,9: O-rings    -   10: Sample space    -   11,12: Gaskets        Explanation of the Symbols Used in FIG. 4    -   13: Cell    -   14: Container for sample    -   15: Container for recovered sample    -   16: Vacuum gauge    -   17: Nitrogen gas    -   18: Vacuum pump ahead of the arrow

BEST MODE FOR CARRYING OUT THE INVENTION

In a cell shown in FIGS. 1 to 3, a trunk 1 has a cylindrical orrectangular prism shape and can transmit an infrared light in the axialdirection. The trunk 1 has one pair of openings at the top and bottom ofthe central portion of the cylindrical or rectangular prism shape, andis connected, at these openings, to a pipe 2 and a pipe 3. As shown inFIG. 4, the pipe 2 present above the cell is connected to an outsidevacuum system and the pipe 3 present below the cell is connected to anoutside sample system.

At the both ends of the trunk 1 of cylindrical or rectangular prismshape are fixed window panels 4 and 5 each made of a material superiorin infrared transmittance. As shown in FIG. 1 and FIG. 2, O-rings 8 and9 are placed between the window panels 4 and 5 and the trunk 1; gaskets11 and 12 are placed between the window panels 4 and 5 and window panelholders 6 and 7; in this state, the window panel holders 6 and 7 aretightened using bolts, whereby the window panels 4 and 5 are fixed tothe trunk 1. As is clear also from FIG. 2, the central portion of thewindow panel holders 6 and 7 are made into a hole so as to allow passingof an infrared light therethrough. Preferred shape of the trunk 1 iscylindrical, because the shape can withstand a high pressure.

A liquid sample is filled in a space 10 formed by the trunk 1 and theinfrared-transmitting window panels 4 and 5; and an infrared lightcoming into the space 10 from one window panel, passes through theliquid sample, leaves the space 10 from other window panel, and reachesan infrared light detection section. The distance sandwiched by thewindow panels 4 and 5, i.e. the length in which an infrared light entersthe liquid sample and leaves it, is called optical path length. Thepreferred optical path length changes depending upon the concentrationof silanol groups in silicon compounds to be measured. A large opticalpath length enables measurement of a silanol group of low concentration,however reduces the strength of cell structure.

As described previously, the present invention aims at measurement ofsilanol group of 0.05 to 0.1 ppm level. Further in the presentinvention, rapid measurement of silanol group is made possible by usinga pressure-resistant cell. In view of the balance between silanol groupconcentration and the pressure resistance of cell, the optical pathlength is preferred to be 5 to 40 mm. Two thin window panels 4 and 5 areinferior in pressure resistance and the thickness of the window panels 4and 5 are preferred to be 2 to 8 mm. The window diameter is preferred tobe 5 to 20 mm.

The material constituting the trunk 1 is preferably Hastelloy orstainless steel because they are superior in corrosion resistance tosilicon compounds, hydrogen chloride, etc. The base material used forthe infrared-transmitting window panels 4 and 5 may be a material whichcan transmit an infrared light of 4,000 to 3,000 cm⁻¹ at which silanolgroup is measured, and for example, potassium bromide, potassiumchloride, sodium chloride, calcium fluoride, germanium, silicon, zincselenide, sapphire and quartz can be used. Of these, preferred aregermanium, silicon, zinc selenide, sapphire and quartz because they aresuperior in strength and hardly give rise to breakage; more preferredare zinc selenide, sapphire and quartz. The material for the O-rings 8and 9 and the gaskets 11 and 12 is preferably, for example, Viton®,Karlez or Teflon®.

In the present invention, in order to remove a very small amount of thewater adhering to the inside of a cell, efficiently in a short time, theprocedure of once reducing the cell-inside pressure to 20 Pa or lowerand then increasing the cell-inside pressure to 0.2 to 1 MPa using a dryinert gas, specifically, for example, nitrogen gas having a watercontent of 0.1 ppm or less, is repeated at least twice prior to fillinga sample into the cell.

The cell of the present invention is connected to a vacuum system and asample system via a pipe 2 and a pipe 3, as shown in FIG. 4.

Before the sample is fed into the cell, the pressure of the cell insideis reduced to 20 Pa or lower using a vacuum pump, then, a dry inert gasis introduced by pressurization. The pressure for the gas introductionis at least 0.2 MPa, however, the upper limit is 1 MPa because too higha pressure causes cell breakage. By conducting such an operation ofpressure reduction and pressurization by inert gas at least twice,preferably at least 5 times, the water in the cell can be removed almostcompletely.

As the pressure-resistant cell which can withstand the above operationand yet can measure a silanol group of very small amount (0.05 to 0.1ppm), a cell having an optical path length of 4 to 50 mm and a windowdiameter of 5 to 20 mm and wherein each window panel has thickness of 2to 8 mm and the trunk is made of a metal such as stainless steel,Hastelloy or the like can be used, for example. A cell using quartz orsapphire as the material for window panel can withstand a pressure of 3MPa. By employing a high pressure in the drying operation of the cellinside, the repeating frequency of pressure reduction and pressureapplication can be reduced.

According to a sample filling system shown in FIG. 4, feeding of thesample into cell can be conducted through a pipe without having samplecontact with atmosphere. A liquid sample can be sent into a cell from asample container via the pipe, by utilizing a pressure differencebetween the sample container and the inside of the cell. After thesample has been filled in the cell, measurement of infrared absorptionspectrum is conducted. After the measurement of infrared absorptionspectrum, an inert gas is introduced to discharge the sample from thecell into a sample-collecting container. By conducting theabove-mentioned operation of pressure reduction and pressure applicationby inert gas to the pipe and the cell inside, the sample adheringthereto can be removed; thereby, a state allowing the measurement ofnext sample is obtained.

Thus, according to the present invention, a very small amount of silanolgroup can be measured at a high accuracy without using an operation ofpassing an inert gas for a long time and cleaning the inside of cellwith a large amount of a sample. Further in the present invention, sincethe feeding of sample into cell is conducted in a closed system, thereis no need of using a glove box.

The cell of the present invention may be used per se. However, in orderto avoid a danger when breakage of window panel has occurred, the cellmay be used in a state that the whole cell is covered with a protectivecase having one pair of windows for infrared transmittance.

The sample to be measured in the present invention is a liquid, organicor inorganic silicon compound, or a silicon compound which is gaseous atordinary temperature, such as hexachlorodisilane or the like. Such asilicon compound may be a single compound or a mixture. Further, a solidsilicon compound soluble in organic solvents, etc. allows formeasurement of infrared absorption spectrum when made into a solution;therefore, the solid silicon compound is included in the sample to bemeasured in the present invention. In the present invention,particularly preferred as the sample to be measured are silicon halides(such as silicon tetrachloride, hexachlorodisilane and the like),alkoxysilanes, etc.

The present invention is described specifically below by way ofExamples.

EXAMPLE 1

As a cell for measurement, there was used a cell having a structure ofFIG. 1 and the following specification.

-   Trunk material: Hastelloy® nickel-based corrosion-resistant alloy,    trunk size: 40 mm (diameter)×40 mm (outer diameter)-   Material for infrared-transmitting window panel: zinc-   selenide (effective diameter for light receiving: 15 mm)-   Optical path length: 2 cm-   Pipe: ¼ inch pipe made of SUS 304-   Analytical instrument and measurement condition: Nicolet Magna 750    FT-IR spectrometer-   Infrared spectrometer: detector=DTGS, beam splitter=potassium    bromide, resolution=4 cm⁻¹, scanning wavelength range=4,000 to 3,000    cm⁻¹, accumulation=32 times

The cell of the present invention was fitted to the infraredspectrometer. The cell was connected to a vacuum system and a samplesystem in position relationships shown in FIG. 4. First, the inside ofthe cell was degassed for about 1 minutes using a vacuum pump until apressure of 20 Pa or lower was reached; then, a dry nitrogen gas wasintroduced until a pressure of 0.5 MPa was reached; this operation ofdegassing and gas-introduction was repeated five times.

In a state that the cell inside was filled with nitrogen gas, abackground spectrum was measured. With cell inside being made vacuumagain, hexachlorodisilane (a product of Toagosei Co., Ltd.) wasintroduced into the cell from a sample container via a pipe to carry outthe first operation of the spectral measurement.

The hexachlorodisilane sample after measurement was discharged from thecell into a sample-collecting container. In order to remove the sampleremaining in the cell and the pipe, degassing was conducted for 10minutes using the vacuum pump; then, nitrogen gas was introduced until apressure of 0.5 MPa was reached. Thereafter, 1-minute pressure reductionand pressure application was repeated 5 times. As a result, theremaining sample was removed and the pressure came down to 20 Pa orlower, and, in the same operation as in the first time,hexachlorodisilane was filled in the cell and spectral measurement wasmade.

In the infrared absorption spectra obtained in the first time and thesecond time, there were observed, at 3,650 cm⁻¹, characteristicabsorption peaks of stretching vibration caused by the OH group ofsilanol group; and their absorbances were 0.0012 and 0.0013,respectively. By comparison with a standard sample (trimethylsilanol) ofknown concentration, the silanol group concentration of first time andsecond time were 3.9 μmol/L and 4.2 μmol/L (0.04 ppm in terms of OHgroup weight).

INDUSTRIAL APPLICABILITY

The method for measurement of silanol group concentration in siliconcompound, according to the present invention can be used for qualitycontrol of the silicon compounds used in electronic material such assilicon nitride film. Further, the pressure-resistant cell formeasurement of infrared absorption spectrum, according to the presentinvention can be used for measurement of infrared absorption spectra ofnot only silicon compounds but also of various other compounds, and ismost suitable for compressed and liquefied gases.

1. A method for measurement of silanol group concentration in siliconcompound by infrared spectrophotometry, which comprises: conducting,prior to filling of a silicon compound in a cell, at least twice each ofa step of keeping the cell inside at 20 Pa or lower and a step ofkeeping the cell inside at 0.2 to 1 MPa, then, introducing the siliconcompound into the cell and measuring the infrared absorption spectrumthereof, to measure the concentration of the silanol group in thesilicon compound and recording the results.
 2. A cell used formeasurement of infrared absorption spectrum capable of withstanding areduced pressure of 20 Pa or lower and a pressure of 0.2 to 1 MPa,comprising a trunk made of stainless steel or nickel-basedcorrosion-resistant alloy and infrared-transmitting window panels andhas an optical path length of 5 to 40 mm, wherein each window panel hasa thickness of 2 to 8 mm.
 3. A cell used for measurement of infraredabsorption spectrum, capable of withstanding an applied pressure of 3MPa or lower, comprising a trunk made of stainless steel or nickel-basedcorrosion-resistant alloy and infrared-transmitting window panels madeof quartz or sapphire and has an optical path length of 5 to 40 mm,wherein each window panel has a thickness of 2 to 8 mm.